1. Field of the Invention
The present invention relates to a method of creating a solid model with a computer-aided design system (hereinafter called "CAD system").
2. Description of Related Art
When the body of a motor vehicle is designed by using a CAD system, its appearance is defined as an overall model by a set of lines curved along curved surfaces of the body. Thus, when a change is to be made to even just a part of the body, every line segment which passes through the part to be changed has to be reprocessed, thereby adding to the designed time.
On the other hand, when shape data is inputted to the CAD system, it is necessary to provide coordinates of apexes and the relationship of connections among the apexes with a view toward defining a three-dimensional model. Hereinafter, for purposes of simplification of terminology, a `solid` is considered an entity existing or represented in three dimensions. Accordingly, the concept of rendering a two-dimensional shape into a three-dimensional shape will be described as `rendering solid` or similar phraseology.
Consider the case where a cross-section taken along line a--a of a front pillar portion of the vehicle shown in FIG. 1a is inputted to the CAD system. In this case, when a glass face 11 of a front is inputted to the CAD system as a reference face, as shown in FIG. 1b, the inclination of a line segment 18 out of input line segments 12 through 18 is rendered positive (+) if its inclination is in a positive direction (clockwise direction) with respect to the reference face. On the other hand, as shown in FIG. 1c, when the above cross-section is seen in a direction opposite to the direction indicated by the arrow of FIG. 1a, i.e., when a cross-section taken along line c--c is inputted to the CAD system, the inclination of the inputted line segment 18 is rendered negative (-) as illustrated in FIG. 1d.
Therefore, there has generally been proposed to make clear the relationship between the direction and the position, for example, a method of using auxiliary lines in the form of matrix, which are indicative of the direction and distance from a reference point, and a method of displaying reference faces common to small parts of a three-dimensional model or the like on the same screen and inputting the relationship between the direction and position with respect to the reference faces or the like so as to create a graphic shape.
However, a practical drawing creation is not always evolved only from drawings projected from a predetermined direction. The turning from the front side to the reverse side of a drawing or vice versa is made or a drawing is created in a state in which it is seen from the reverse side, thereby causing a possibility that the positive and negative signs of the position and angle are reversed, and a false creation of the drawing is made unless the operator is experienced. In addition, such a data inputting method has the problem that much labor is required upon inputting the data and the operator is apt to make errors upon inputting the data.
When it is desired to construct a solid shape as a product on the CAD system, individual graphic shapes are defined as elements. Thereafter, the operator adds the relationship between graphic shapes to the CAD system; shapes which the operator desires to create must also be decomposed into elements and the so-decomposed elements must be defined. In addition, many operations are required to define individual elements. The relationship of correspondence between the graphic shapes is also represented in the form of lines, and the solid shape is not defined as a solid including faces.
On the other hand, when it is desired to create a solid model in the CAD system, the solid model is seen from a plurality of directions indicated by arrows, and respective intersecting points in the plurality of directions and line segments defined along the plurality of directions are determined. They are then connected to each other to create faces. However, when the intersecting points and the line segments are determined, a plurality of line segments (or points) exist in a mixed state, thereby causing difficulty in grasping or recognizing shapes. In addition, since the solid model is held as data about ridgelines, a large number of design steps are required.